1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus and, more particularly, to an optical scanning device that scans a scanning surface with light and an image forming apparatus that includes the optical scanning device.
2. Description of the Related Art
Image forming apparatuses that utilize a laser as a light source are often used for forming electrophotographic images. For that purpose, the image forming apparatus is equipped with an optical scanning device that scans the surface of a photosensitive element with a light flux (a scanning light flux) that has been emitted from the light source and deflected by an optical deflector in order to form a latent image on the photosensitive element.
As the light source, a semiconductor laser is used in general. An edge emitting laser has been most used, however in recent years, a vertical cavity surface emitting laser (also known as VCSEL) has been introduced. In the edge emitting laser, approximately only four up to eight is the limit of the number of emitting elements in its array configuration. Meanwhile, the surface emitting laser makes it possible to further increase the number of emitting elements in its array configuration. For that reason, the surface emitting laser is now expected to be the light source that realizes higher density and faster speed in the image forming apparatuses.
However, in the image forming apparatuses, the light intensity of the scanning light flux may vary with a variation in temperature and/or time. Thus, a density irregularity may be induced in an image finally outputted (an output image).
Hence, an optical scanning device with the conventional edge emitting laser performs auto power control (APC) that monitors the light emitted backward from the edge emitting laser to control the variation in the light output. However, the surface emitting laser has a structure not allowing the light emitted backward therefrom. Therefore, it is required a way of controlling the light output (the light intensity) different from the conventional APC in the optical scanning device that uses the surface emitting laser. As such a way of controlling the light output suitable for the apparatus or device that uses the surface emitting laser, there is proposed some methods that a part of the light flux emitted from the surface emitting laser is split by an optical element, such as a beam splitter or half mirror, and directed to an optical detector, so that the APC is performed according to the output from the optical detector. These methods are disclosed by Japanese Patent Application Laid-open No. 2005-274678 and in Japanese Patent Application Laid-open No. 2007-298563.
In the optical scanning device disclosed by Japanese Patent Application Laid-open No. 2005-274678, however, the light output of a laser beam directed to the scanning surface necessarily decreases by an amount of the laser beam directed to the optical sensor. Therefore, the disclosed method has a disadvantage in realizing the high speed image forming.
Also in the optical scanning device disclosed by Japanese Patent Application Laid-open No. 2007-298563, the proposed method has a disadvantage in a point that the accuracy of the APC decreases, when the splitting device has any machining error.